Astroviruses cause 2-17 percent of the diarrhea episodes that require medical care in children. Our current understanding of the molecular epidemiology, seroprevalence, and immune response to astrovirus has been derived from limited studies from a variety of locations. Additional data are needed to determine if the prevention strategy of vaccine development is warranted. This study will describe attributes of human astroviruses (HAstVs) in two well-defined surveillance studies. The hypotheses of this study are that multiple HAstV antigenic types and strains are present simultaneously in a community, that type-specific immunity provides protection from symptomatic infection, and that HAstV illness results in an economic burden sufficient to warrant development and implementation of protective strategies. The specific aims of this study are: 1) To characterize the antigenic and genetic diversity of HAstVs among children in two prospective surveillance studies including hospitalized children in three locations of the U.S. and children in Mexico City under a community-based surveillance. 2) To characterize the immune response to HAstV among naturally infected children from these populations by measuring type- specific anti-HAstV antibody. 3) To determine the role of serum antibody in protection of children from symptomatic infection, and to determine whether the protection is type-specific or group-specific. 4) To determine if the burden of disease is sufficient to develop a vaccine by evaluating the economic impact of severe HAstV diarrhea among children in three U.S. cities over two years of surveillance. Information regarding type variability, genomic variability, molecular epidemiology, and type-specific immune response to astrovirus are necessary to develop preventive measures including immunization strategies.